This invention relates to a high frequency circuit for starting and ballasting gas discharge lamps. More particularly, the invention relates to a high efficiency, high frequency electronic inverter circuit for operating one or more electric discharge lamps.
One significant feature or aspect of the present invention is the provision of a unique oscillator-inverter ballast circuit that produces multiple high frequency modes of operating frequency in which the inverter frequency of operation automatically changes during each period of the 60 Hz AC supply voltage in a manner so as to regulate the lamp discharge current.
The prior art has employed a variety of techniques for energizing and ballasting electric discharge lamps. The early ballast circuits were energized by means of a DC voltage or a 60 Hz AC voltage and, in the case of the AC supply voltage, necessitated the use of a rather large magnetic ballast transformer. These early ballast circuits were characterized by a relatively poor efficiency caused in part by the relatively large power losses in the ballast system itself. More recently it has been proposed to improve the efficacy of a system for energizing discharge lamps by operating the lamps at a high frequency, generally in a range of 15 KHz to 50 KHz.
One such high frequency ballast system is described in U.S. Pat. No. 4,220,896 by D. A. Paice. This patent discloses a high frequency resonant feedback inverter energized from a DC power source for operating a discharge lamp via a ballast circuit including an inductor and capacitor connected in series. The discharge lamp is connected across the capacitor and the inverter frequency is adjusted to regulate the inverter AC output voltage level and to maintain almost unity power factor at the input to the ballast filter.
U.S. Pat. No. 4,259,614 by T. P. Kohler employs a push-pull transistor oscillating inverter for energizing a pair of discharge lamps via a ballast circuit comprising a series resonant LC circuit that determines the inverter oscillation frequency. The peak lamp current is sensed and used to control the inverter frequency so that the frequency is reduced as the lamp current is increased, thereby limiting the power dissipation of the circuit.
Another high frequency inverter oscillator is illustrated in U.S. Pat. No. 4,017,785 by L. J. Perper which provides a supplemental DC power supply connected so as to supplement a fluctuating main DC supply to maintain continuous oscillator operation and to substantially reduce the peak AC line current.
A second unique aspect of the present invention is the provision of a novel magnetic impedance transformer for coupling the inverter oscillator to the discharge lamp or lamps. A high frequency leakage reactance transformer is used to provide an automatic reduction in the heater power or current supplied to the discharge lamp filament electrodes once the lamp ignites thereby producing a so-called auto-heat mode of operation. At the same time, the leakage reactance of the transformer also produces a ballast function to protect the discharge lamp.
The use of a small high frequency leakage inductance transformer for coupling a high frequency inverter-oscillator to a discharge lamp is shown in U.S. Pat. No. 3,579,026 in the name of F. W. Paget. This patent discloses a full wave rectifier which supplies an unfiltered rectified direct current to a high-frequency oscillator inverter that is coupled to a pair of discharge lamps via the high frequency leakage transformer. The inverter oscillation frequency is dependent on the applied voltage. The lamps have preheatable electrodes energized by secondary windings of the leakage transformer which are tightly coupled to the transformer primary winding. A low frequency ballast utilizing a manually adjusted variable reactance to control the lamp discharge current is described in U.S. Pat. No. 2,458,277 by G. T. K. Lark et al. In the Lark et al ballast the heating current for the lamp filaments is reduced as the lamp discharge current is increased. And Canadian Pat. No. 670,797 discloses a discharge lamp ballast circuit including a novel arrangement of transformer windings by means of which the heating voltage for the lamp electrodes is higher before lamp ignition than it is after ignition.